A variety of devices for separating fluid mixtures with hollow fiber membranes have been described. See for example, U.S. Pat. Nos.: 4,961,760; 4,929,259; 5,000,763; 5,013,331; 5,013,437; 5,071,552; 5,160,042; 5,282,964; 5,702,601; 5,284,583 and 5,779,897.
Typically, the separation process is carried out in a module fabricated from semi-permeable membranes. Such membrane separations are based on relative permeabilities of various components of the fluid mixture, resulting from a gradient of driving forces, such as pressure, partial pressure, concentration and temperature. Such selective permeation results in the separation of the fluid mixture into retentate, i.e. slowly permeable components, and permeate portions, i.e. faster migrating components.
In boreside feed processes the feed fluid is introduced into the open bores of the hollow fibers and one or more components permeate through the walls of the hollow fibers into the region outside the fibers. The fluid which selectively permeates through the fiber membrane wall is removed from the shellside of the membrane, while the non-permeable fluid is removed from the non-permeate region.
Obtaining proper flow and distribution of the permeate fluid on the shellside of the fibers is a problem associated with boreside feed. During separation, the high permeate flow rate may result in excessive shellside pressure drops. Additionally, the uncontrolled flow of the permeate fluid on the shellside of the membrane may cause localized areas of high concentration or partial pressure of the permeate fluid, thus resulting in inefficient or ineffective separation of the fluid mixture.
The efficiency of the fluid separation process is determined by the properties of fluid mixture, the membrane material and its structure. The productivity of the membrane device is proportional to the surface area of the membrane material packed in the device, while the separation efficiency of the device inversely depends on the thickness of the membrane material. Generally this is achieved by providing the membrane as hollow fibers of substantial length and small diameter, arranged parallel to one another. However, decreasing the diameter of the long fibers can result in increased back pressure. The efficiency of separation drops further with the use of thinner capillaries or with highly permeable capillaries with asymmetric wall structures. Other methods to decrease the shellside pressure drop, such as larger fiber size, shorter device length and decreased fiber packing density result in increased cost and/or decreased module productivity.
Therefore, current membrane devices containing long hollow fibers with small diameters are costly and inefficient when the pressure drop is large and thus not commercially viable for meeting current uses. Thus, there is a need for improved and cost-effective devices comprising fiber membranes that are capable of operating at acceptable levels of separation productivity.